Some players are quite eager to start quests to get them over with. However, it seems a lot of people don't understand that when you finish the countdown for the teleporter/transporter, the quest in progress no longer can be joined, even if there are slots remaining. This is a feature in the Japanese version of the game as well.

Do not use the teleporter/transporter early unless you are confident in the already-joined players' abilities. Especially for Urgent Quests like the Mining Base.